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Share-Based Payments (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of stock option activity and related information
A summary of the Company’s stock option activity and related information follows:
 
Shares
 
Weighted-Average
Exercise Price
 
Remaining
Contractual Life
(in years)
Aggregate
Intrinsic
Value
(in thousands)
Outstanding at December 31, 2016
2,024,468

 
$
4.41

 
8.73
$
841

Granted
1,303,025

 
1.64

 
 
 
Exercised
(140,400
)
 
0.28

 
 
 
Cancelled or forfeited
(491,297
)
 
5.10

 
 
 
Outstanding at December 31, 2017
2,695,796

 
$
3.16

 
8.55
$
146

Exercisable at December 31, 2017
1,267,285

 
$
4.35

 
7.56
$
146

Vested and expected to vest at December 31, 2017 (1)
2,275,796

 
$
3.45

 
8.33
$
146

(1)Represents the number of vested options, plus the number of unvested options expected to vest. The Company adopted ASU 2016-09 as of January 1, 2017 and elected an accounting policy to record forfeitures as they occur.
Summary of status and changes of unvested restricted stock
A summary of the status of unvested restricted stock as of December 31, 2017 and 2016, and changes during the year ended December 31, 2017 are presented below:
 
Restricted
Stock
 
Weighted-Average
Grant Date
Fair Value
Unvested at December 31, 2016
22,150

 
$
11.43

Vested
(17,720
)
 
11.43

Unvested at December 31, 2017
4,430

 
$
11.43
Summary of the status of restricted stock unit activity
A summary of the status of restricted stock units is presented below:
 
Restricted
Stock Units
 
Weighted-Average
Grant Date
Fair Value
Unvested at December 31, 2016
3,333

 
$
4.09

Vested
(3,333
)
 
4.09

Unvested at December 31, 2017
—

 
$
—
Estimation of fair value of stock option granted using Black-Scholes option-pricing model assumptions
The fair value of each stock option granted to employees and directors was estimated on the date of grant using the Black-Scholes option-pricing model based on the assumptions noted in the following table:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Risk-free interest rate
1.88-2.04%
 
1.23-2.38%
 
1.42-1.92%
Expected dividend yield
—%
 
—%
 
—%
Expected term (in years)
5.3-6
 
5.5-6
 
5.75-6
Expected volatility
75.4-86.66%
 
71.44-73.42%
 
69.06-74.11%
The fair value of each non-employee stock option is valued on grant date and revalued as it vests using the Black-Scholes option-pricing model based on assumptions noted in the following table:
 
Year Ended December 31,
 
2017
 
2016
 
2015
Risk-free interest rate
1.50-2.32%
 
1.08-2.38%
 
1.19-2.26%
Expected dividend yield
—%
 
—%
 
—%
Expected option life (years)
10
 
10
 
10
Expected stock price volatility
74.4-84.31%
 
69.92-92.09%
 
67.24-92.40%